The Boston Collaborative Drug Surveillance Program which started in 1966 consists of the routine systematic, standardized recording of a wide variety of well-specified items of information in a setting of medical or psychiatric wards. The primary data recording is performed by specially trained nurses. These people, while full-time employees of the surveillance program, nonetheless are an integral part of the staff of the monitored ward. The monitor, following a common protocol and using a standard set of self-coding forms, obtains the data by interviewing the attending physician and patient, where appropriate, and by examining the medical record. The data collected on the patient fall into four broad categories of information: (1) drug utilization prior to entry into hospital, (2) patient characteristics, including diagnosis as recorded on discharge, (3) treatment regimens administered during the hospitalization, and finally, (4) drug-attributed or routinely observed clinical events occurring during hospitalization. These data are edited by human review of the source documents and subsequently more thoroughly by computer programs applied to the master tape files undergoing updating periodically. These data permit the description of drug utilization patterns, quantitation of known and suspected drug effects, and the discovery and evaluation of previously unknown drug effects. The data also permit the study of a wide range of other clinical, epidemiological and non-pharmacological problems